The present invention relates to an optical amplifier using the optical amplification of a rare earth-doped fiber, and relates to an optical isolator and an optical circuit which are used for the above optical amplifier.
The above optical isolator includes characteristics that a ray penetrates in the direction from the input side to the output side with little loss but scarcely penetrates in the direction from the output side to the input side and the basic structure thereof is well known.
A conventional optical amplifier is discussed in, for example, "1990 Spring national convention record, the institute of electronics, information and communication engineers C-273". In this reference an optical amplifier comprising an Er-doped fiber, a pumping light source, an optical circuit which can couple pumping light from the pumping light source and an optical signal from a transmission fiber, and optical isolators without polarization dependency which are connected to the input side end and the output side end of the Er-doped fiber respectively is disclosed. The used Er-doped fiber is an Alcodoped 1.55 .mu.m dispersion sifted fiber with an Er concentration of 66 ppm and a refractive index difference .DELTA. of 1.2%. The maximum gain at a pumping light input of 25 mW with a wavelength of 1.48 .mu.m is 27.5 dB for forward single pumping or 33.0 dB for bidirectional pumping.
To increase the pumping efficiency of the above Er-doped fiber in the above prior art, the Er doping concentration, the core diameter for doping Er, and the refractive index of the core are controlled. However, no consideration is given to connection losses at the connecting portions with the optical isolator at the input side end and the output side end of the above Er-doped fiber.
The outline of the optical fiber amplifier is indicated in, for example, M. Horiguchi; Kogaku (Optics), Vol. 19, No. 5 (May, 1990), pp. 276-282.